Allow z -b to do fuzzy matching

If `z -b foo bar` finds no exact match in the path to the current directory,
it tries a "fuzzy" substitution with a frecent directory. For example, if
we are in `~/github/jekyll/test` and run `z -b jek gh`, it will try to
substitute the entire `jekyll` path component with `gh`. The result will
be equivalent to running `z ~/github gh test`.

README.md

@@ -319,6 +319,7 @@ New option `"-b"` can quickly go back to a specific parent directory in bash ins
 - **(No argument)**: `cd` into the project root, the project root the nearest parent directory with `.git`/`.hg`/`.svn` in it.
 - **(One argument)**: `cd` into the closest parent starting with keyword, if not find, go to the parent containing keyword.
 - **(Two arguments)**: replace the first value with the second one (in the current path).
+  If simple substitution does not work, falls back to fuzzily replacing path components.
 
 Let's start by aliasing `z -b` to `zb`:
 
@@ -338,6 +339,11 @@ Let's start by aliasing `z -b` to `zb`:
 # substitute jekyll with ghost
 ~/github/jekyll/test$ zb jekyll ghost
   => cd ~/github/ghost/test
+
+# same as above, but fuzzy
+~/github/jekyll/test$ zb jek gh
+  => z ~/github/ gh /test
+    => cd ~/github/ghost/test  # Assuming that's the most frecent match
 ```
 
 Backward jumping can also be used with `$_ZL_ECHO` option (echo $PWD after cd), which makes it possible to combine them with other tools without actually changing the working directory (eg. ``ls `zb git` ``). 

z.lua

@@ -1759,7 +1759,7 @@ function cd_backward(args, options, pwd)
 			end
 			return nil
 		end
-	else
+	elseif nargs == 2 then
 		local test = windows and pwd:gsub('\\', '/') or pwd
 		local src = args[1]
 		local dst = args[2]
@@ -1770,10 +1770,21 @@ function cd_backward(args, options, pwd)
 		if not start then
 			return pwd
 		end
+
 		local lhs = pwd:sub(1, start - 1)
 		local rhs = pwd:sub(ends + 1)
-		return lhs .. dst .. rhs
+		local newpath = lhs .. dst .. rhs
+		if os.path.isdir(newpath) then
+			return newpath
+		end
+
+		lhs = lhs:gsub("[^/]*$", "")
+		rhs = rhs:gsub("^[^/]*", "")
+		return z_cd({lhs, dst, rhs})
 	end
+
+	io.stderr:write("Error: " .. Z_CMD .. " -b takes at most 2 arguments.\n")
+	return nil
 end
 
 
@@ -1922,7 +1933,7 @@ function main(argv)
 	if options['--cd'] or options['-e'] then
 		local path = ''
 		if options['-b'] then
-			if Z_INTERACTIVE == 0 then
+			if #args > 0 or Z_INTERACTIVE == 0 then
 				path = cd_backward(args, options)
 			else
 				path = cd_breadcrumbs('', Z_INTERACTIVE)